This disclosure describes means to provide the Sagnac secure fiber optic communication systems similar to those described in detail in U.S. Pat. Nos. 5,223,967, 5,274,488, 5,311,592, 5,422,772, and 5,455,698 using a single installed optical fiber. The teachings in those patents are incorporated into this disclosure by reference as though fully set forth below.
This invention relates generally to fiber optic communications and more particularly to Sagnac interferometer based fiber optic communication systems that utilize counterpropagating optical beams to impress data on a loop by means of phase modulation. In order to make a system of this type practical for general use, a flexible, single fiber configuration is needed to support the base of currently installed optical fiber.
The need for high bandwidth secure communication systems that are amenable to usage in networks and which minimize the need for encryption is becoming increasingly acute as more sophisticated systems come on line. Generally, encryption reduces the data throughput of a given system by an order of magnitude when compared to non-encrypted throughput. The need for additional data throughput is expected to continue into the indefinite future with networks supporting ever greater numbers of users demanding higher and higher bandwidth. The advent of fiber optic telecommunication systems has opened up a new era of low cost, high bandwidth systems that are enabling a host of new applications. Sagnac secure fiber optic communication systems offer the prospect of transmitting this data securely, but heretofore, such has not been adaptable to the installed single fiber, single light band links.
There is provided by this invention a Sagnac interferometer based secure fiber optic communication system that allows transmission of data securely along conventional telecommunication fiber cables. This invention is designed to include many of the advantages of the conventional loop configuration of Sagnac secure fiber optic communication systems while minimizing dispersion and fiber compatibility problems by using a single fiber configuration between the transmitting and receiving ends of the system.
The system includes a light source that is generally broadband with a low coherence length. The light source injects light into a fiber beamsplitter that is used to generate counterpropagating light beams in a Sagnac loop. The loop includes two facing fiber beamsplitters connected together at both inner legs, with one of the output legs of the second beamsplitter being connected to a transmission fiber line that ends with a phase modulator followed by a mirror at a fixed distance. Formatted data may be transmitted by impressing relative phase differences between the counterpropagating light beams in a manner similar to that described with respect to other secure Sagnac interferometric communication systems. Optimum performance depends on appropriate choices for critical lengths in the system.
Therefore, it is an object of the present invention to provide a secure fiber communications system that can use conventional fiber optic communications fibers as installed for non-secure telecommunications.
Another object is to essentially eliminate the need to install special optical fibers and links to install a secure fiber communications system.
Another object is utilize installed zero dispersion single mode fibers in a secure fiber communications system whether they be designed for 1300 or 1550 nm light.
These and other objects and advantages of the present invention will become apparent to those skilled in the art after considering the following detailed specification including the drawings wherein: